FIG. 3 shows a prior art semiconductor laser device having a double-hetero structure and a current blocking layer having a stripe ridge. In FIG. 3, reference numeral 1 designates a p type GaAs substrate. N type GaAs current blocking layer 2 is disposed on p type GaAs substrate 1. Lower cladding layer 3 comprising p type Al.sub.0.48 Ga.sub.0.52 As is disposed on current blocking layer 2. P type Al.sub.0.15 Ga.sub.0.85 As active layer 4 is disposed on lower cladding layer 3. N type Al.sub.0.48 Ga.sub.0.52 As upper cladding layer 5 is disposed on active layer 4. N type GaAs contact layer 6 is disposed on upper cladding layer 6. A current injection groove 7 portion 7 of cladding layer penetrates a stripe ridge formed by GaAs substrate 1 and current blocking layer 2. Reference numeral 8 designates the stripe ridge portion. P side electrode 10 and n side electrode 11 are disposed on p type GaAs substrate 1 and n type contact layer 6, respectively. The crystal growth of the stripe ridge 8 is achieved by liquid phase epitaxy (hereinafter referred to as "LPE"). The crystal growth speed greatly depends on the crystal-lographic orientation of the crystal surface. That is, the crystal growth speed at the side surface of ridge is faster than at top flat portion. The rate of growth on the top of the ridge is suppressed by the fast growth at the ridge side surface. As a result, the thickness of the active layer 4 can be reproducibly made less than 0.05 micron.
The light generated at active layer 4 spreads out to the lower and upper cladding layers 3 and 5. That light has a certain distribution area determined by the light propagation characteristics of the laser cavity, the parameter of principal importance being the thickness of active layer.
When the thickness of active layer 4 becomes as thin as 0.05 microns, the area of light distribution is broadened without the increase of threshold current. In other words, too thin an active layer causes the extreme increase in the threshold current. Therefore, that thickness should not to be much thinner than about 0.05 microns. Therefore, because the peak intensity of the light distribution is reduced as the distribution area of light spreads out at a predetermined light output, the light output level at which the catastrophic optical damage (hereinafter referred to as "COD") increases the facet as the active layer 4 becomes thinner. That is, the enhanced light output power is realized by the thinning of the active layer 4, and the thickness of the active layer 4 can be reliably controlled in the stripe ridge configuration.
However, the ridge configuration affects not only the active layer 4 but also the lower cladding layer 3. As a result, the thickness of the lower cladding layer 3 at the top portion of the ridge is made thin so that the thickness of the lower cladding layer 3 may be made too thin. Furthermore, when growing the p type lower cladding layer 3 on the n type current blocking layer 2, some melt used back of the current blocking layer 2 into the melt for growing the lower cladding layer 3 occurs at the top portion of the ridge. As a result the carrier concentration of the cladding layer 3 is reduced compared to the situation in which those layers are grown in totally planar regions. Thus, the pn junction between the current blocking layer 2 and the lower cladding layer 3 may not be perfectly produced. In such case the leakage current flowing directly from the current blocking layer 2 on the ridge to the upper cladding layer 5 is increased, and the threshold current and the operation current of laser oscillation are increased. These current increases reduce the yield in the production of these semiconductor laser devices.